The present invention relates to a novel cyclosilane compound, a solution composition for forming a silicon film and a process for forming a silicon film.
Heretofore, thermal chemical vapor deposition (CVD), plasma CVD and optical CVD which use monosilane gas or disilane gas have been used to form an amorphous silicon film or polysilicon film. Generally speaking, thermal CVD (J. Vac. Sci. Technology, vol. 14, pp. 1082, 1977) and plasma CVD (Solid State Com., Vol. 17, pp. 1193, 1975) have been widely used for the formation of a polysilicon film and an amorphous silicon film, respectively, and these films have been used for the production of solar cells and liquid crystal display elements having a thin film transistor.
However, in the respect of a process for forming a silicon film in accordance with these CVD methods, further improvement on the following points has been awaited. (1) Since a gas-phase reaction is used, silicon particles generated in a gas phase contaminate an apparatus and form foreign matter, thereby reducing production yield. (2) Since raw materials are gaseous, it is difficult to obtain a uniformly thick film on a substrate having an uneven surface. (3) Productivity is low due to a low film formation speed. (4) A complex and expensive high-frequency generator and vacuum apparatus are needed for plasma CVD.
In the respect of raw materials, since gaseous silicon hydride which has high reactivity and toxicity is used, it is difficult to handle it and an airtight vacuum apparatus is required. This apparatus is bulky and expensive and a vacuum or plasma system consumes a huge amount of energy, thereby boosting the costs of a product.
A method of coating liquid silicon hydride without using a vacuum system has recently been proposed. JP-A 1-29661 (the term xe2x80x9cJP-A xe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a process for forming a silicon-based thin film by liquefying a gaseous raw material on a cooled substrate, adsorbing it to the substrate and reacting it with chemically active atomic hydrogen. However, this process involves such problems that a complex apparatus is required and that it is difficult to control the thickness of a film because silicon hydride as a raw material is vaporized and cooled successively.
JP-A 7-267621 discloses a process for coating a substrate with low-molecular weight liquid silicon hydride. This process has such problems that it is difficult to handle silicon hydride as the system is instable and that it is difficult to obtain a uniform film thickness when it is applied to a large-area substrate because silicon hydride is liquid.
Meanwhile, English Patent GB-2077710A reports an example of solid silicon hydride polymer but a coating film thereof cannot be formed because it is insoluble in a solvent.
Further, JP-A 9-237927 discloses a process for forming a silicon film by coating a substrate with a polysilane solution and thermally decomposing polysilane to liberate a silicon film in order to produce a solar cell. However, it is difficult to obtain an amorphous or polycrystal silicon film having excellent electrical properties from a silicon compound containing carbon because thermal decomposition or optical decomposition by exposure to ultraviolet radiation causes a large amount of carbon to remain in the film as an impurity.
Further, JP-A 60-242612 discloses a process for forming a silicon deposition film by thermal CVD, the process comprising forming a gaseous atmosphere of a cyclic silane compound represented by the following formula and a halogen compound in a deposition chamber where a substrate is placed and providing heat energy to these compounds to form a deposition film containing silicon atoms on the substrate. 
(where n is 3, 4 or 5, R is H or SiH3).
The above publication enumerates compounds represented by five chemical formulas as the cyclic silane compound represented by the above formula. However, it is silent about identification data on these cyclic compounds and a production process therefor. Polysilanes can be generally produced from monomers having a structural unit by the following methods, for example: (a) one in which a halosilane is dehalogenated and polycondensed in the presence of an alkali metal equivalent in weight to the halogen atom (so-called Kipping method, J. Am. Chem. Soc., vol. 110, pp. 2342, (1988), Macromolecules, vol. 23, pp. 3423, (1990)); (b) one in which a halosilane is dehalogenated and polycondensed by electrode reduction (J. Chem, Soc., Chem, Commun., pp. 1161, (1990), J. Chem. Soc., Chem. Commun., pp. 896, (1992); (c) one in which a hydrosilane is dehydrogenated and polycondensed in the presence of a metal catalyst (JP-A 4-334551); (d) one in which a disilane crosslinked by biphenyl is anionically polymerized (Macromolecules, vol. 23, pp. 4494, (1990)); and (e) one in which a cyclic silicon compound substituted by a phenyl group or alkyl group is synthesized by one of the above methods and induced into a hydro-substituted material or halogen-substituted material by a known method (for example, Z. Anorg. Allg. Chem., vol. 459, pp. 123, (1979)). These cyclosilane halide compounds may be synthesized by known methods (for example, Mh. Chem. vol. 106, pp. 503, (1975)), (Z. Anorg. Allg. Chem. vol. 621, pp. 1517, (1995)), (J. Chem. Soc., Chem. Commun., pp. 777, (1984)) As described above, a polysilane is generally synthesized by a polycondensation reaction but the step of purifying a synthesized polymer is needed as a salt is by-produced by polycondensation. It has recently been disclosed in xe2x80x9cMacromolecules, vol. 27, pp. 2360, (1994))xe2x80x9d that poly(phenylnanomethylpentasilanylene) 2 is synthesized by the anionic ring-opening polymerization of phenylnanomethylcyclopentasilane 1. The ring-opening polyaddition reaction of this cyclic monomer by-produces nothing unlike the above polycondensation reaction and is excellent as a means of synthesizing a high-purity polysilane. This ring-opening addition polymerization is particularly preferred in application fields which require high purity such as electronic materials. However, since the above monomer has the carbon atom of a methyl group (Me) or phenyl group (Ph) bonded to a silicon atom, even when the above polysilane is thermally decomposed polycarbosilane containing a carbon atom is formed, while silicon for semiconductors cannot be obtained. 
It is an object of the present invention to provide a novel cyclosilane compound consisting of silicon atoms and hydrogen atoms.
It is another object of the present invention to provide a solution composition suitable for forming a silicon film, particularly an amorphous silicon film or polycrystal silicon film on the surface of a substrate based on the new fact that the above cyclosilane compound of the present invention provides a high-purity and solvent-soluble polysilane by the radical ring-opening addition polymerization of a cyclopentasilane without by-producing a salt.
It is still another object of the present invention to provide a process for forming a silicon film having uniform thickness and a mirror surface on a substrate in a short period of time.
Other object and advantages of the present invention will become apparent from the following description.
According to the present invention, firstly, the above objects and advantages of the present invention are attained by silylcyclopentasilane represented by the following formula (A): 
According to the present invention, secondly, the above objects and advantages of the present invention are attained by a solution composition for forming a silicon film which contains cyclopentasilane represented by the following formula (B) and silylcyclopentasilane represented by the above formula (A): 
According to the present invention, thirdly, the above objects and advantages of the present invention are attained by a process for forming a silicon film on the surface of a substrate, the process comprising the steps of:
(1) preparing the above solution composition for forming a silicon film of the present invention;
(2) coating the surface of a substrate with this solution composition to form a coating film; and
(3) heating this coating film to form a silicon film.
Further, according to the present invention, there is also provided spiro[4.4] nonasilane represented by the following formula (C) and having the function of radically ring-opening polymerizing cyclopentasilane like the silylcyclopentasilane of the present invention: 